The mechanics of human injury is a complex process whereby external forces are transmitted to the musculo-skeletal system, the system responds in the manner of displacements and rotations, stresses are induced in the tissues, and at some point certain tissues are disrupted. When the external forces are not constant, as is usually the case, prediction of the likelihood of injury and the severity of the induced stresses is increasingly difficult. This proposal presents a project investigating the injury mechanisms and lower extremity dynamic response to the forces normally encountered in snow skiing. Snow skiing has been chosen for study because the lower-extremity injury rate is high, because the forces transmitted to the leg by the ski and the leg response can be measured, and because safety devices can reduce the high rate of injury associated with the sport. The specialized equipment needed for this study has been developed by the proposer and is not available elsewhere. The proposed study is essential to progress on the skiing injury problem from two standpoints. First, the outlined problems include measurement of forces on the leg, EMG and leg response during skiing and particularly during falling when severe loading conditions occur. These field measurements identifying the injury environment are accompanied by laboratory measurements on the test subjects to clarify how the lower extremity responds to dynamic loading. This is called the "Identification Problem" for lower extremity dynamics. Dummy tests for evaluation of safety devices are also to be further developed. The second point necessitating this work is the correction of the common misconceptions that skiing forces are small compared to typical tibia fracture strength values, and that muscle forces need not be considered in the safety problem. These conceptual errors are widespread, even entering the safety standard organizations, and can only be corrected through research of the type outlined here. The errors have major implications in the design of safety devices, in the setting of standards, and in the instructions given individual skiers.